Cathode ray tube

ABSTRACT

The present invention relates to a cathode ray tube suitable for use with a flat type cathode ray tube in which a material from which a secondary electron is emitted by the impingement of an electron beam is exposed on a surface of an area of a first panel portion (2) having on its inner surface formed a phosphor screen on which the electron beam from an electron gun impinges and an insulative material which forms a second panel portion (3) is exposed on an inner surface of the second panel portion (3) opposing the first panel portion (2) wherein the secondary electron is accumulated to present a predetermined high potential state so that an electric field is prevented from being disturbed in a path of the electron beam.

This is a continuation of application Ser. No. 678,690, filed Nov. 21,1984 now abandoned.

TECHNICAL FIELD

The present invention relates to a cathode ray tube and moreparticularly to a flat type cathode ray tube.

BACKGROUND ART

In a cathode ray tube such as a cone type television tube the whole ofthe inner surface of its envelope from the periphery at the electronbeam emitting end of an electron gun to a phosphor screen there is aninner conductive film made by coating carbon thereon. A constant highvoltage is applied to the inner conductive film whereby the electronbeam emitted from the electron gun can be stably directed to thephosphor screen.

The above structure was provided because when the glass surface of thecathode ray tube proper or the surface of the insulating material facesthe path of the electron beam, an unstable electric charge is stored onthe surface of the insulating material which may cause a disturbance ofthe electric field in the path of the electron beam to thereby cause adisplacement of the scanning position of the electron beam and hence togenerate flicker of distortion in the picture. Previously, the presentassignee proposed a flat type cathode ray tube consisting of a flatglass envelope 1 whose view in perspective is shown in FIG. 1 and whosecross-sectional view is shown in FIG. 2. This glass envelope 1 is formedof first and second panel portions 2 and 3 which are opposed to eachother to establish a flat space 7 therebetween. The panels are bondedtogether by frit-sealing and a funnel portion 4 is similarly bonded toone side of the first and second panel portions 2 and 3 by frit-sealing.The funnel portion 4 is so formed that its open end 4a of large diameteris contacted and sealed to the first and second panel portions 2 and 3while to its open end 4b of small diameter there is welded a neckportion 5 within which an electron gun 6 is located.

The first and second panel portions 2 and 3 comprise, as shown in theirexploded and perspective view in FIG. 3, main faces 2a and 3a whichoppose each other and peripheral side faces 2b and 3b which extend fromthree side edges other than side edges bonded to the funnel portion 4.The end surfaces of the peripheral side faces 2b and 3b, which opposeeach other, are frit-bonded to establish the flat space 7 between boththe panel portions 2 and 3. In order that the flat funnel-shaped spaceof the funnel portion 4 communicate with the flat space 7, to the sideedge portions of the panel portions 2 and 3 where there exist noperipheral side faces the large diameter opening 4a is contacted andsealed by frit-bonding.

On the inner surface of the face 2a of the first panel portion 2, thereis formed a conductive layer 8 made of a vapor deposited aluminum filmand thereon a phosphor screen 9 is formed by, for example,electrodeposition. A protective film 10 is coated on the phosphor screen9 and a transparent conductive layer 11 made of a vapor deposited filmis coated on the protective layer so as to cover the whole inner surfaceof, for example, the first panel portion 2. Further, on the innersurface of the funnel portion 4 there is an inner conductive film 13made of a carbon coating film or the like. An anode button 14 forapplying a high voltage is provided through the funnel portion 4, forexample, at one side which is electrically connected to the innerconductive film 13. From this anode button through the inner conductivelayer 13 to the transparent conductive film 11 and hence to the phosphorscreen 9 and the high voltage electrode of the electron gun 6, there isapplied is a high anode voltage. The face 2a of the first panel portion2 is so curved that the phosphor screen 9 formed on its inner surfaceopposite the axis of the electron gun 6, as it approaches the tip end ofthe envelope 1, namely the side opposite to the side near the locationof the electron gun 6, it comes near or intersects the tubular axis sothat the electron beam emitted from the electron gun 6 impinges on thephosphor screen 9 at about its center when the electron beam is notdeflected. The electron beam emitted from the electron gun 6 isdeflected by a horizontal and vertical electromagnetic deflection means17 provided on, for example, the peripheral portion near the weldedportion of the funnel portion 4 and the neck portion 5 such that itscans the phosphor screen 9 over a predetermined area horizontally andvertically. A light image emitted from the phosphor screen 9 by theexcitation caused by the impingement of the electron beam thereon isviewed from the side of, for example, the face 3a of the second panelportion 3.

Even in the flat type cathode ray tube which is formed by integrallybonding the first and second panel portions 2 and 3 to the funnelportion 4 to which the neck portion 5 is welded, it is desired thatsimilar to the cathode ray tube of an ordinary television receiver, theelectron beam path at the side of the phosphor screen should besurrounded by the conductive film to which the high constant voltage isapplied as described above to prevent the electric field for theelectron beam path from being disturbed.

Therefore, in such a flat type cathode ray tube, on the inner surface ofthe second panel portion 3, a transparent conductive film 12 isevaporated over the whole area thereof to which the high voltage isapplied through the anode button 14. In case of such structure, thetransparent conductive films 11 and 12 respectively coated on the innersurfaces of the first and second panel portions 2 and 3 are suppliedwith the high voltage by electrically connecting the conductive films tothe inner conductive film 13 of the funnel portion 4. However, in theportions between the respective conductive films 11, 12 and 13 there areconnecting surfaces formed by the frit-bonding of the respective panelportions 2, 3 and the funnel portion 4 so that the conductive films 11,12 and 13 can not be electrically connected. Accordingly, it isnecessary that in this case after the respective portions 2, 3 and 4 arefrit-bonded, the respective conductive films 11 and 12 are electricallyconnected to the inner conductive film 13. This electrical connection iscarried out such that before electron gun 6 is inserted into the insideof the neck portion 5 bonded to the funnel portion 4, a special deviceis inserted into the envelope from the rear opening end of a neckportion 5 and the conductive material such as carbon paint or the likeadhered to the tip end of the above device is coated across thefrit-bonded portions of the funnel portion 4 to the first and secondpanel portions 2 and 3 and bridges parts of the conductive film 13 andfilms 11 and 12 to thereby provide connecting portions 15 and 16 whichelectrically couple the conductive film 13 with films 11 and 12.

In such a flat type cathode ray tube, in order to surround entirely theperiphery of the electron beam path directed to the phosphor screen,namely, the entire periphery of the flat space by a conductive film, theexpensive transparent conductive film is coated on both the innersurfaces of the first and second panel portions 2 and 3. Especially forthe second panel portion 3, this procedure is not only expensive butalso complicated. Providing two coupling conductive portions 15 and 16made of carbon paint or the like across the frit-bonded portions is verycomplicated and prevents its mass-production.

In view of the above defects of the prior art, the present inventionseeks to reduce the area on which the transparent conductive film iscoated as much as possible to thereby reduce the cost, simplify theworkability and also increase the reliability.

DISCLOSURE OF INVENTION

According to the present invention, it was found that even if aconductive film is not formed on the entire periphery of the electronbeam path at the side of the phosphor screen and a predetermined voltageis applied thereto, the electric field in the electron beam path can bestabilized.

That is, in the present invention, on the whole or at least a part ofthe electron beam scanning area on which the electron beam impinges atthe side of the first panel portion, a layer of material, whichgenerates a secondary electron beam when the electron beam impingesthereon, is provided, whereby no transparent conductive film is providedat least on the second panel portion. And, even if the surface ofinsulating material which forms the panel portion itself, for example,the glass surface is exposed, when the electron beam impinges upon thephosphor screen, namely, the cathode ray tube begins to be driven,secondary electrons are emitted so that this secondary electron beam isaccumulated so as to cover the surface of the insulative material coatedon the inner surface of the second panel portion, thus applying aconstant potential to the inner surface thereof.

BRIEF DESCRIPTION OF DRAWINGS

FIGS. 1 and 2 are respectively a perspective view and a longitudinallycross-sectional view of a flat type cathode ray tube useful forexplaining the present invention,

FIG. 3 is an exploded perspective view of a tube envelope thereof and

FIG. 4 is a longitudinal cross-sectional view of an embodiment of acathode ray tube according to the present invention.

BEST MODE FOR CARRYING OUT THE INVENTION

An embodiment of a cathode ray tube, particularly a flat type cathoderay tube according to the present invention will be described withreference to FIG. 4. Also in this embodiment, the envelope 1 of thecathode ray tube is formed such that the first and second panel portions2 and 3 and the funnel portion 4 to which the neck tube 5 incorporatingtherein the electron gun 6 is welded are integrally bonded byfrit-sealing and the like. In FIG. 4, like parts corresponding to thosein FIG. 2 are marked with the same reference numerals and will not bedescribed. Particularly in this invention, the transparent conductivefilm 12 described in connection with FIG. 2 is not deposited on theinner surface of the second panel portion 3 but the insulative materialwhich forms the panel portion 3, for example, the glass surface isdirectly exposed and opposed to the side of the phosphor screen 9.

Further in accordance with the present invention, the whole area onwhich the electron beam b from the electron gun 6 impinges, namely, theportion corresponding to the scanning area of the electron beam, or atleast a part of the surface layer thereof is formed in material of whichthe secondary electron emitting ratio is relatively high. For example,when the transparent conductive film 11 is formed so as to cover thephosphor screen 9, the transparent conductive film is formed of materialhaving a relatively high secondary electron emitting ratio, for example,an evaporated film of a composite oxide film (ITO) of In and Sn. Thistransparent conductive film 11 is formed on, for example, the wholeinner surface of the first panel 2 similarly as mentioned before andelectrically coupled with the inner conductive film 13 of the funnelportion 4 by the coupling conductive layer 15 which is coated after thefrit-sealing as mentioned similarly to FIG. 2. Through this transparentconductive film 11, the high voltage can be applied to the phosphorscreen from the anode button 14. This transparent conductive film 11 isformed on the protective film 10 formed on the phsophor screen 9. Inpractice, the surface of the phosphor screen 9, namely, the surface ofthe electrodeposited film of phosphor powder has very small concave andconvex portions. On the other hand, the protective film 10 and thetransparent conductive film 11 formed on the above surface are bothformed sufficiently thin so that the phosphor screen 9 can beefficiently excited by the electron beam. As a result, the surface isnot fully covered with the transparent conductive film 11 so thatmicroscopically a part of the protective film 10 or a part of phosphorof the phosphor screen 9 is exposed. Accordingly, in this case, theprotective film 10 is formed of a silicon oxide having a relatively highsecondary electron emitting ratio, namely, SiO, SiO₂, a mixture thereofor an intermediate form thereof. Alternatively, the phosphor itselfwhich forms the phosphor screen 9 is made of sulfide having a highsecondary electron emitting ratio.

According to the configuration of the invention as mentioned above,since the material surface from which the secondary electron is emittedis exposed in the scanning area of the electron beam by the impingementof the electron beam thereon, at the same time when the cathode ray tubeis starting to be driven, the secondary electrons are emitted, advancedtoward, for example, the inner surface of the second panel portion 3opposing thereto and accumulated therein. Since the potential of thesecondary electrons are high, the inside of the tube can be held at astable state of a predetermined high voltage in a short period of time.

While in the above embodiment the transparent conductive film 11 isformed on the whole inner surface of the first panel portion 2, thetransparent conductive film 11 may be formed only on, for example, thephosphor screen and a path for supplying a high voltage to the phosphorscreen may be formed of a carbon layer and the like. In this case, evenwhen a part of the first panel portion 2, namely, glass or an insulativematerial forming the same is exposed, a predetermined electrificationstate is formed by the accumulation of the secondary electrons generatedfrom the above electron beam scanning area to thereby prevent theelectric field from being disturbed in the path of the electron beam.

As described above, the inside of the cathode ray tube is stabilized bythe emission of the secondary electrons. The reason why the inside ofthe cathode ray tube is stabilized was determined that since the spacewithin the tube envelope is the flat space, in a relatively short timeof period after the driving of the cathode ray tube is started, theglass exposed portion within the tube envelope is covered with thesecondary electrons and a stable electrification state, namely, anequilibrium state is established.

As set forth above, according to the configuration of the invention, noconductive film is deposited at all on the inner surface of the secondpanel portion 3 so that an optical image from the phosphor screen isviewed from, for example, the side of the panel portion 3, without theexpensive transparent conductive film on the inner surface of the panelportion being present. Further, since it is possible to avoid theformation of the coupling conductive layer 16 which, as shown in FIG. 2,is used to couple the inner conductive film 13 formed on the innersurface of the funnel portion 4 to the conductive film formed on theinner surface of this panel portion 3, reliability is made high andworkability can be increased.

While in the above embodiment the present invention is applied to thecathode ray tube of so-called reflection type in which the optical imageformed on the phosphor screen is viewed from the opposite side of thepanel having the phosphor screen, the present invention is not limitedto a cathode ray tube of such reflection type. It is needless to saythat the present invention can be applied to a cathode ray tube of theso-called tranparent type in whcih the conductive layer 8 on the sideof, for example, the phosphor screen 9 is formed as the transparentconductive film and the light emission of the phosphor screen is viewedfrom the outer side of the inner surface 2a of the panel portion with asimilar effect being achieved.

In addition, it is clear that the present invention is not limited tothe above embodiment but can be applied to cathode ray tubes of variouskinds in which the first and second panel portions are opposed to eachother with the similar effect being achieved.

We claim:
 1. A cathode ray tube comprising:a first panel portion havinga phosphor screen on its inner surface, a transparent second panelviewing portion in opposed relation to said first panel, a funnelportion including a necked down portion, said funnel portion beingbonded to said first and second panel portions, an electron gun disposedin said necked down portion, said first panel portion having aconductive undercoated layer and a material comprising anelectrodeposited phosphor film which produces substantial secondaryelectron emission upon bombardment by an electron beam, said secondpanel portion including an uncoated surface on its inner surfacedirectly facing said first panel portion, and being subject to electronbombardment from electron emission from said material, whereby electronbombardment by the electron gun on said first panel portion causessecondary electrons to accumulate on said inner surface of said secondpanel portion to create a stable electrical field within said tube in ashort time.
 2. A cathode ray tube according to claim 1, wherein saidsecond panel portion is formed of transparent glass and the phosphorscreen of said first panel portion is viewed through said second panelportion.
 3. A cathode ray tube of flat type according to claim 1,wherein said first and second panel portions are formed substantiallyflat.